1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a method for non-destructively detecting a laser optical path through an IC package.
2. Description of the Related Art
IC devices are formed from a die of active semiconductor devices. The die can be mounted in a hybrid circuit, printed circuit board (PCB), or a package. For environmental protection, the die may be covered by a passivation layer. However, a package is more typically used since it also dissipates heat and provides a lead system for electrical connections. There are many different types of packages including through-hole, surface mount device (SMD) dual/quad, and SMD area array packages.
FIG. 1 is a perspective view of a dual in-line package (DIP) (prior art). It is common for a package body or lead frame 100 to have a die attach area 102. The die 106 has electrical contact pads on its top surface. Inner leads 108 connect pads on die top surface to outer leads or lead frames 110. Once the inner leads are bonded to the lead frames, the package is sealed with ceramic, in a metal can, or in a polyimide. Epoxy resins are also a common choice. Glass beads are commonly mixed in with the epoxy to reduce strain in the epoxy film during changes in temperature.
Optical beam induced current (OBIC) is a semiconductor analysis technique performed using laser signal injection. The technique induces current flow in the semiconductor sample through the use of a laser light source. This technique is used in semiconductor failure analysis to locate buried diffusion regions, damaged junctions, and gate oxide shorts.
The OBIC technique may be used to detect the point at which a focused ion beam (FIB) milling operation in bulk silicon of an IC must be terminated. This is accomplished by using a laser to induce a photocurrent in the silicon, while simultaneously monitoring the magnitude of the photocurrent by connecting an ammeter to the device's power and ground. As the bulk silicon is thinned, the photocurrent increases as the depletion region of the well to substrate junction is reached. FIB milling operations are terminated in a region below the well depth, so the device remains operational.
FIG. 2 is a partial cross-sectional view of an optical pathway formed through glass beads in an epoxy compound sealant. The laser marking, or labeling by material ablation, of integrated circuits is routinely performed using a CO2 laser, directly on the package surface after the epoxy mold compound is applied. It has been reported in technical literature that an integrated circuit can be damaged using a laser for identification marking on the outside of the package. An optical path can be inadvertently formed by the alignment of the glass particles in the epoxy mold compound. During the marking process, a laser can follow the optical path to the die. If the laser energy is sufficient, metal lines on the die can be melted and shorted, or active semiconductor regions of the die can be damaged. As IC packages continue to decrease in size, the epoxy compound thicknesses inherently decrease, and the statistical probability of forming inadvertent optical paths through the glass particles increases.
It should be noted that the use of epoxy compounds with glass beads is an industry standard. The size and distribution of glass balls determines the flow characteristics, uniformity, and void formation. The glass balls “balance” other ingredients in the epoxy that have been developed and improved for years. The removal of glass balls from epoxy compounds would be a major complication for packaging assemblers and epoxy suppliers. Further, the use of materials other than glass balls would require the requalification of many IC packages.
The optical pathways caused by glass balls can potentially be identified through metallurgical cross-sectioning in multiple locations, and detailed examination of the cross-sections. However, this cross-sectioning process is tedious and labor intensive. Nondestructive investigation methods such as x-ray and acoustic microscopy cannot detect these optical pathways.
It would be advantageous if a method existed to detect the susceptibility of an IC within a package to laser identification marking damage, in an effort to improve the quality and reliability of packaged integrated circuits.